Hall field plasma accelerators (or thrusters) with closed electron drift employ electrons discharged from a separate cathode and directed toward an anode by an applied electric field (E) through an applied magnetic field (B) which is generally orthogonal to the applied electric field and in which the electrons collide with atoms of a gas or propellant to create a plasma which consists of approximately equal number of electrons and ions which are accelerated out of the accelerator/thruster by the applied electric field. Generally the Larmor radius .rho..sub.e of the electrons is much smaller than the characteristic length L of the accelerator so the electrons tend to move in a helical path about the magnetic lines as they move from line to line azimuthally and drift generally toward the anode. The ions in contrast have a Larmor radius .rho..sub.i which is much greater than the characteristic length L so the path of the ions is largely unaffected by the magnetic field.
The thrust and power density of the accelerator increases with increasing mass flow rate of the plasma gas. The upper limit on the mass flow rate is set by the requirement to minimize the number and frequency of collisions between ions and neutral atoms. Such collisions are undesirable because they thermalize the plasma and divert the accelerating ions from their primary path, causing some of them to strike the containment walls which leads to wall heating and sputtering, all contributing to a loss of efficiency and reduction of accelerator life. The mean distance an ion travels before colliding with a neutral atom is known as its mean free path .lambda..sub.in. It is proportional to 1/(nQ) where n is the number of atoms per unit volume and Q their collisional cross-section. To minimize the number of collisions it is required to have the characteristic length L smaller than the mean free path.
Thus if an accelerator or thruster can be made with a very small characteristic length L, .lambda..sub.in can be made concomitantly smaller so that n can be increased. More atoms per unit volume (n) generally means more ions and thus more power from a smaller device.
One construction known as a thruster with anode layer (TAL) has a very short acceleration zone: the characteristic length L is short so it has a high number n and operates at high power density. Because of the high power density which generally results in high heat loads the anode is typically made of materials such as graphite or high melting point metals to withstand the elevated temperature. In another construction, a stationary plasma thruster (SPT), the characteristic length L is much larger because the anode is set deep within its dielectric discharge chamber. Since the length L is greater it must have a lower n and so operates at a lower power density.
Separately, plasma physics equations dictate that in coexisting mutually orthogonal electric and magnetic fields, the magnetic field lines approximate the equipotential contours in the plasma. This relationship of equipotentials and magnetic field lines is distorted by the presence of electric and/or magnetic conductors. Thus in the SPT type of device, for example, the fringing magnetic field dictates the electric field distribution within the plasma which may create undesirable ion trajectories leading to reduced performance, diverging ion beams and reduced lifetime. In the TAL type device the consequences of Maxwell's equations and small L results in high energy electrons impacting the anode.